Humans are continually inhaling environmental fungi. When the host immune system is competent, the inhaled fungi are cleared away from the lung by host defense mechanisms. But in immunocompromised individuals, the environmental fungi (e.g., Aspergillus fumigatus) sometimes cause infection. Pathogenic fungi possess various mechanisms to invade the host. A. fumigatus is no exception in possessing several virulence factors and defense mechanisms against host immune attack.One of the virulence factors is secondary metabolite. A. fumigatus produces a variety of secondary metabolites, and the fungal products in culture supernatant have a strong apoptosis-inducing activity to macrophages and alveolar epithelial cells. These data suggest that A. fumigatus is equipped with special projectile weapons for destroying host physical barriers and immunological barriers in lung.The fungal cell wall is an easy target for the host to recognize the pathogen. One of the fungal cell wall components, beta- (1,3) -glucan, is a major fungal PAMP (pathogen-associated molecular pattern), which is recognized by one of the pattern recognition receptors, dectin-1. The interaction induces activation of transcription factors and production of proinflammatory cytokines in the host cell. However, beta-glucan of A. fumigatus is strongly exposed to the surface only during the "swollen-conidia" phase. In the hyphal phase, the fungus is covered with "armor", i.e., other cell wall components to minimize the exposure of the beta-glucan structure. These findings suggest that A. fumigatus evades the recognition and the attack from host by masking beta-glucan. A. fumigatus has clever mechanisms to defend itself and to attack the host immune system.